Block interrupts during HandleParallelMessages().

 Block interrupts during HandleParallelMessages().
 
 As noted by Alvaro, there are CHECK_FOR_INTERRUPTS() calls in the shm_mq.c
 functions called by HandleParallelMessages().  I believe they're all
 unreachable since we always pass nowait = true, but it doesn't seem like
 a great idea to assume that no such call will ever be reachable from
 HandleParallelMessages().  If that did happen, there would be a risk of a
 recursive call to HandleParallelMessages(), which it does not appear to be
 designed for --- for example, there's nothing that would prevent
 out-of-order processing of received messages.  And certainly such cases
 cannot easily be tested.  So let's prevent it by holding off interrupts for
 the duration of the function.  Back-patch to 9.5 which contains identical
 code.
 
 Discussion: <14869.1470083848@sss.pgh.pa.us>